Semiconductive pressure sensors are now in widespread use. A common form includes a semiconductive chip, typically of silicon, that includes a thin membrane or diaphragm at one of its two major surfaces, formed by etching a cavity in the opposite other major surface. One or more piezoresistive strain gages (typically piezoresistors) are formed in the diaphragm to sense changes in stresses as the diaphragm is exposed to the pressure being sensed. The sensor may be either of the absolute type, in which the pressure on one side of the diaphragm is maintained at a reference value, or of the differential type, in which the pressures on opposite sides of the diaphragm vary independently and their difference is measured. It is also advantageous in such sensors to include in the semiconductive chip additional circuitry to process the relatively weak signals detected by the strain gages into stronger signals for more convenient use in the control system of which the pressure sensor generally is part.
For various well-recognized reasons, including greater sensitivity and better reproducibility, it is advantageous that the piezoresistive strain gages be formed in monocrystalline semiconductive material. To this end, it is the usual practice for the semiconductive chip that houses the strain gages and the additional circuitry to be monocrystalline.
Hitherto, to form a semiconductive chip that includes a diaphragm that is monocrystalline, the practice has been to start with a monocrystalline semiconductive chip of uniform thickness and to form the thin diaphragm on one surface by thinning the chip locally by etching a cavity in the opposite surface of the semiconductive chip.
It is recognized that such two-side processing has some serious disadvantages. In particular, because the final thickness of the diaphragm is determined by the small difference of two larger values, the processing needed to insure a prescribed diaphragm thickness tends to be difficult.
Accordingly, it has been recognized that it would be advantageous to have a single-sided processing method in which all of the critical processing steps are conducted solely on the same surface of the semiconductive chip that houses the strain gages.
A single-sided process has been described in U.S. Pat. No. 4,766,666 that issued on Aug. 30, 1988. However, it is characteristic of the process described therein that it results in a diaphragm that is polycrystalline with the subsequent disadvantages that such a structure entails.